A conventional imaging sensor array is formed from a plurality of photosensitive pixel elements typically arranged in an array forming rows and columns. As the size of pixel elements has decreased and associated sensor circuitry has been integrated into multi-level integrated circuit devices, the photosensitivity of the pixel elements including photodiodes has correspondingly required improvement to counteract the effects of increased light scattering, low u-lens gain, and low fill factor. For example the fill factor, as is commonly referred to in the art, is the ratio of a photodetector area divided by the pixel area. For example, a photodetector typically includes an array of photosensitive elements, often referred to as pixels, including spacing between the pixel array rows and columns which are not active in photosensing. A higher fill factor is one method to increase photosensitivity.
Recently elevated PIN photodiodes with high fill factors have been used to increase photosensitivity. One problem with elevated PIN photodiodes has been related to vertical leakage current that occurs at metal and intrinsic amorphous silicon forming Schottky junction interfaces. For example, typically intrinsic amorphous silicon sensor material, also referred to as an I-layer creates charge carriers upon illumination with light through a P+ doped amorphous silicon portion (layer). The charge carriers are then selectively collected by a Schottky diode electrode formed of an N+ doped amorphous silicon portion over a metal portion. The metal portion collects the charge from the N+ doped amorphous silicon portion for transport to detector or readout electronics, for example including CMOS transistors.
During the formation of the of the photodiode sensors, prior art processes have frequently left metal portions of the Schottky diode electrode in contact with the I-layer. As a result, vertical leakage current upon application of a bias voltage to the Schottky diode electrode degraded charge collection and photosensitivity of the PIN photodiodes. For example, under reverse bias conditions, a leakage current resulting from contact injection occurs at an interface of the intrinsic amorphous silicon layer (I-layer) and the metal portion or back contact.
While several PIN sensor elements have been disclosed in the prior art, the teachings as disclosed in Theil et al., in U.S. Pat. No. 6,018,187 and Mei et al., in U.S. Pat. No. 6,288,435 are incorporated herein by reference.
For example, in Mei et al., an approach to address the vertical leakage current problem has been to extend the N+ amorphous silicon layer, in a mushroom shaped contact electrode to cover metal/I-layer interfaces. One problem with such structures has been vertical leakage current due to contact of the active layer (I-layer) with the conductive portion of the electrode.
Another problem with the method of the prior art in forming thin film PIN photodiodes is that mushroom shaped contacts increase the electrode size which thereby increases an active pixel size. The increased size of the electrode including the extension of the N+ doped amorphous silicon layer to encapsulate the metal portion has the offsetting effect of decreasing the fill factor thereby lowering photosensitivity. In addition, the method is time consuming and costly in terms of throughput and processing costs in that at least two photolithographic patterning steps must be performed to form the extended N+ doped amorphous silicon layer.
There is therefore a need in the image sensor art for an improved PIN sensor including rear electrode formation to improve a thin film PIN photodiode fill factor while avoiding the formation of Schottky junctions producing a vertical leakage current.
It is therefore an object of the invention to provide an improved PIN sensor to improve a thin film PIN photodiode fill factor including a rear electrode and method of forming the same while avoiding the formation of Schottky junctions producing a vertical leakage current, in addition to overcoming other deficiencies and shortcomings of the prior art.